I accidentally sit on my iphone 11 would it damaged it or not?

Unless you really jammed on it when you sat down, you probably didn't do anything to it. I always have mine in my back pocket and I have sat on it a few times. I usually take it out of my pocket before I sit down, but have forgotten a few times. Anyway, if you're not seeing any damage when you look at it and nothing is cracked and everything is working like it should, you most likely didn't do anything to it and you should be okay.

Maybe/maybe not...if you have not noted any physical damage, and it still works as advertised, consider yourself fortunate.